<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    /* 2n就是2的倍数 */
    /* odd是选择奇数 */
    /* even是偶数 */
    ul li:nth-child(even){
        background-color: pink;
    }
</style>
<body>
    <div>
        <ul>
            <li>666</li>
            <li>666</li>
            <li>666</li>
            <li>666</li>
            <li>666</li>
            <li>666</li>
        </ul>
    </div>
</body>
</html>